A-Level Investigation into Bernoulli Principle

I'm a UK based student undertaking A-Level Physics. I am now in my second year completing my majour investigation. We are allowed to study and investigate any physics phenomina(sp) of our choice, I have decided upon a ball in a jet looking into the effects of Bernoulli.

One of the things stressed by my tutor is that the investigation must provide the opportunity for taking a list of results, varying a factor, and then taking another set of results and looking at the relationship between the two.

I have completed a few initial experiments using some simple apparatus:

Air Jet

Boss

Clamp

Ping Pong Ball

Meter Ruler

The Air Jet has four variable jet speeds. It's lowest speed I recorded at 19ms. For the variable in the experiment I decided to change the surface of the ball.

Firstly I used a non-tampered Ping Pong Ball with a smooth surface. The ball gained an average height of 221mm.

Secondly I prepared a Ping Pong Ball dipped into PVA glue and then coated with sand, I believed this would increase the aerodynamic turbulance in the airflow around the ball and therefore cause an increase in the height it gained, I was wrong. The ball again averaged a height close to the non-tampered ball, 218mm.

Thirdly I drilled a series of holes into a ball (3mm in diameter), the holes were spaced randomly. I estimate that around half of the initial surface area of the ball was lost, there were many holes drilled. When I then tried to gain a reading the ball persistantly "fell out" of the air jet and refused to comply with the motion the conventional Ping Pong Ball adopted.

I have a few questions that I hope you can help me with.

1. Can you suggest any other thing I could vary to measure and graph the change in height of the ball?

2. Why does the conventional Ping Pong Ball "hover", that is, maintain a constant height. I'm aware that both the downward and upward forces acting upon the ball must be equal. I belive I can explain this by using either F=ma or P(Momentum)=mv, am I correct?

3. Finally, why does the ball with holes drilled into it fail to be suspended in the air jet?